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排序方式: 共有147条查询结果,搜索用时 15 毫秒
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Tree encroachment into rough fescue (Festuca campestris) grassland has been identified as an ecological concern on the Cypress Hills plateau in southeastern Alberta, Canada. A combination of field sampling (109 transects), a dendrochronological assessment (1361 trees), and a time series analysis of remotely sensed images from five different time periods (1950–2002) were used to determine the extent and rate of tree encroachment and forest development. Tree cover increased by 768 ha (~51%) between 1950 and 2007, representing 10% of the study area, from 1502 ha of pre-1950 forest. Post-1950 tree invasion also created an ingress zone of 750 ha (~10% of study area) based on field transects. Forest cover increased at a consistent rate of 14.3 ha/year. Lodgepole pine (Pinus contorta var. latifolia) was the most common tree species associated with encroachment. Invasion based on the number of established trees occurred at an exponential rate of 3.1%/year after 1890, with density increased by filling spaces adjacent to and between trees within the grassland vegetation. The rate of tree establishment increased to 4.4%/year after 1980, suggesting a change occurred in environmental conditions. Annual atmospheric temperatures increased 0.55 °C from 1929 to 2005 (P < 0.001), with winter (December–February) and spring (March–May) temperatures accounting for most of the increase (P < 0.001), whereas summer (June–August) temperatures slightly decreased (P < 0.050, 0.34 °C) and precipitation increased (P < 0.005, 30 mm). Cattle and elk (Cervus elaphus) grazing was not considered a primary factor for explaining tree encroachment. Based on multidimensional scaling, lodgepole pine establishment was associated with warmer spring temperatures and greater fall (October–November) precipitation. A landscape devoid of wildfires, combined with greater moisture availability, and a longer frost-free season is likely conducive to the sustained establishment of coniferous trees and forest development within the Cypress Hills fescue grassland ecosystem. Long-term conservation of the fescue grasslands could be possible by reintroducing fire. 相似文献
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Smith KE Wall R Howard JJ Strong L Marchiondo AA Jeannin P 《Veterinary parasitology》2000,88(3-4):261-268
The insecticidal effects of the phenylpyrazole, fipronil, and a pyrethroid, beta-cyfluthrin, on larvae of the blowfly Lucilia sericata were determined in laboratory assays. When first stage larvae of L. sericata were reared on homogenized pig liver which had been treated with known amounts of test compounds, both fipronil and beta-cyfluthrin induced significant levels of mortality compared to acetone and water controls. However, fipronil was approximately 10 times more toxic than beta-cyfluthrin to L. sericata larvae following ingestion. Beta-cyfluthrin had little effect on mortality until concentrations of approximately 0.5 ppm were reached. In contrast, fipronil effected L. sericata mortality at a concentration of 0.05 ppm and 100% mortality was reached by 0.5 ppm. The lethal concentration (LC50) value for beta-cyfluthrin was 1.56 ppm as compared to 0.14 ppm for fipronil. Following contact of first and third stage larvae with cloth impregnated with known amounts of test compound, the mortality profiles of fipronil and beta-cyfluthrin were similar. At short contact times, the LC50 values for fipronil were lower than those for beta-cyfluthrin. However, at the highest contact time evaluated for the first stage larvae, 300 s, there was a reversal in this trend. The results suggest that the phenylpyrazole fipronil may represent a new potential insecticide for development against blowfly strike of sheep. 相似文献
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Strong RM 《Science (New York, N.Y.)》1916,44(1129):238-239
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Strong RM 《Science (New York, N.Y.)》1912,36(927):443-445
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Christel C. Kern Brian J. Palik Terry F. Strong 《Forest Ecology and Management》2006,230(1-3):162-170
We evaluated ground-layer plant diversity and community composition in northern hardwood forests among uncut controls and stands managed with even-age or uneven-age silvicultural systems. Even-age treatments included diameter-limit cuttings (20-cm diameter at 30-cm stem height) in 1952 and shelterwood removals in 1964. Uneven-age treatments included three intensities of selection harvest (light, 20.6 m2/ha residual basal area after harvest; medium, 17.2 m2/ha residual basal area; and heavy, 13.8 m2/ha residual basal area) that were applied in 1952, 1962, 1972, and 1982. All treatments were winter logged over snow pack. In 1991, plant diversity and community composition were examined. Species richness for spring ephemerals ranged from 1 to 6 species/150-m2, spring ephemeral diversity (Shannon's Index of Diversity) averaged 0.57 ± 0.04 and evenness averaged 0.45 ± 0.03. Summer flowering species richness ranged from 1 to 18 species/1-m2, with an average diversity of 0.71 ± 0.07 and evenness of 0.42 ± 0.03. We found no significant differences among treatments for any of these variables, although power to detect a difference (at p = 0.05) was low in all cases (0.15–0.55) due to high variance and low replication. Community composition was not significantly different among the treatments, for either spring (p = 0.09) or summer (p = 0.79) flora. Few exotic species were found in any treatment. Lack of exotic invasions and minimal differences in plant diversity or composition among treatments may be due, in part, to the negligible amount of soil disturbance that resulted from winter logging. While other (unmeasured) ecosystem components may differ among these silvicultural treatments, our results suggest that ground-layer plant communities in northern hardwood ecosystems are either resistant to change or have recovered within the 40 years since disturbance in the even-age treatments and within 10 years since disturbance in the uneven-age treatments. 相似文献
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Strong LC 《Science (New York, N.Y.)》1945,101(2633):608-609
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Analysis of samples collected within the United States after the Chinese atmospheric nuclear weapons tests of 26 September and 17 November 1976 indicates that the radiation dose to the thyroid from iodine-131 in milk was predominant. A U.S. population dose to the thyroid of 68,000 man-rads was calculated for the iodine-131 fallout. The four excess thyroid cancers that are estimated to occur as a result of the September test during the next 45 years will be masked by the 380,000 cases of thyroid cancer which are expected to occur in the United States from all causes during the same interval. 相似文献